Tavern displays quests that can be completed for different rewards.
The quests will reset daily or can be refreshed any time with (10 Gems*number of not locked quests).
1★ and 2★ quests can be completed immediately for free.
Other quests can be completed immediately by spending Gems.
Locking a quest will "save" it from being auto refreshed the next day. by locking a quest you also lock 1 of you refreshes, so if your maximum is 5 quests, 1 locked, you will get 1 locked and 4 new quests the next day.
Another way to "save" quests is to complete the quest, but not redeem it, this will lock your heroes to the quest, but will grant you a new quest the next day. if you maximum is 5 quests, 1 completed, the next day will be 1 completed and 5 new quests.